1. NUCLEAR CHEMISTRY

2. Section 22-1: The Nucleus Objectives Explain what nucleons are.
Explain what a nuclide is, and describe the different ways it can be written.
Define nuclear binding energy.
Explain the relationship between nucleon number and stability of nuclei.

3. The Nucleus The nucleus is composed of nucleons
Protons
Neutrons
A nucleus is characterized by two numbers
mass number (A; total # of nucleons)
atomic number (Z; number of protons)
ZAE

4. 1327Al total number of nucleons is 27 total number of protons is 13
the number of neutrons is 14
in nuclear chemistry, an atom is referred to as a nuclide.

5. Subatomic Particles one atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom

6. Mass of 24He The actual mass is 4.00260 amu.
Why the difference in mass? mass defect

7. Einstein’s Equation Energy and mass can be interconverted E = mc2
E- energy, m-mass, c-speed of light
When protons & neutrons are packed together to form a nucleus, some of the mass is converted to energy and released.
Mass is lost

8. Nuclear Binding Energy
Nuclear Binding Energy – the energy released when a nucleus is formed from protons and neutrons.
Can also be thought of as the amount of energy required to break apart the nucleus. of an existing atom.
The higher the binding energy, the more tightly the nucleus is held together.

9. Binding Energy Curve graph peaks at A=56
the higher the BE, the more stable the nucleus
mass number of 56 is maximum possible stability

10. How Many Neutrons? Stable nuclides have certain characteristics
The number of neutrons in a nucleus can vary as we have seen
Range limited by the degree of instability created by:
having too many neutrons
too few neutrons
Stable nuclei do not decay spontaneously
Unstable nuclei have a certain probability to decay

11. Nuclear Stability Facts
265 stable nuclides
For light elements (Z  20), Z:N ratio is ~1
Example: helium-4 (2 neutrons, 2 protons)
Z:N ratio increases toward 1.5 for heavy elements
Example: lead-206 (124 neutrons, 82 protons)
For Z > 83 (bismuth), all isotopes are radioactive
This is due to repulsive forces of the protons

12. Nuclear Stability Facts
Stable nuclei tend to have an even number of nucleons.
Out of 265 stable nuclides, 159 have even numbers of both protons and neutrons.
Only 4 nuclides have odd numbers of both.

13. The most stable nuclides are those having: 2, 8, 20, 28, 50, 82, 126
Protons, neutrons or total nucleons
Examples:
Sn (Z=50) has 10 isotopes; In (Z=49) & Sb (Z=51) have only 2 isotopes
Pb-208 has a double magic number (126n, 82p) & is very stable
“Magic numbers” of protons or neutrons which are unusually stable

14. Nuclear Reactions
Nuclear Reaction – a reaction that affects the nucleus of an atom.
Unstable nuclei undergo spontaneous changes that change their number of protons and neutrons.
They also give off a large amount of energy and increase their stability in the process.

15. Nuclear Reactions
In a nuclear reaction, the total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation.
Example:
49Be He C n

16. 49Be He C n
Note that when the atomic number changes, the identity of the element changes.
Transmutation – a change in the identity of a nucleus (element) as a result of a change in the number of its protons.

17. Sample Problem Identify the product that balances the following
nuclear reaction:
84212Po ? He

18. Sample Problem Identify the product that balances the following
nuclear reaction:
84212Po Pb He

19. Classwork
Problems 1-3, page 704

20. Homework
Page 724
Problems: 31, 33 and 40

21. Section 22-2
Radioactive Decay

22. Radioactivity Objectives
Define the terms radioactive decay and nuclear radiation.
Describe the different types of radioactive decay.
Define the term half-life, and how it relates to stability.

23. Radioactivity
The spontaneous decay of an unstable nucleus into a more stable nucleus.
Energy is released.
Certain isotopes are just not stable and will spontaneously decay.

24. Types of Radioactive Decay
All elements with 84 or more protons (Polonium) are unstable, and will undergo radioactive decay.
Naturally occurring radioactive isotopes decay in four primary ways:
Alpha particle emission
Beta particle emission
Gamma radiation emission
Positron emission

26. Alpha Emission
Alpha particle (a) – is two protons and two neutrons bound together and emitted from the nucleus.
They are helium nuclei with a charge of +2.
It has no electrons.
Represented by the symbol: 24He or 24a
Restricted to heavy elements: Ex. uranium

27. Alpha Emission
Process which is effective to lose a lot of mass form the element.
Example:
84210Po Pb a
The atomic number decreases by two (a new element) and the mass number decreases by four.

28. Alpha Emission 92235U 90231Th + 24 (or 24He)
Quick way for a large atom to lose a lot of nucleons

29. Beta Emission
Beta particle – is essentially an electron that’s emitted from the nucleus: -10b
In the nucleus, a neutron is converted (decayed) into a proton and an electron. 01n p b
The electron is emitted as a beta particle.
Represented by the symbol: -10e or -10b
A good way to decrease the number of neutrons.

30. Beta Emission
By decreasing the number of neutrons you improve the neutron/proton ratio.
Example:
53131I Xe b
The mass number stays the same in going from I-131 to Xe-131 but the atomic number increases by 1. Loss of neutron!

31. Beta Emission
1940K Ca b
Identity of atom changes

32. Gamma Emission
Gamma rays – are high energy electromagnetic waves emitted from the nucleus.
There is no mass change with gamma emission, only radiation.
Usually occurs immediately following other types of decay.
Not shown in a balanced nuclear reaction.

33. Gamma Emission
An example is cobalt-60 (Co-60) which gives off a large amount of gamma radiation.
Co-60 used in the radiation treatment of cancer.

34. Electromagnetic Radiation
Electromagnetic radiation is a form of energy that can pass through empty space
It is not just a particle, and it is not just a wave. It may be both.

35. Electromagnetic Radiation
Gamma rays are similar to x-rays – high energy, short wavelength radiation.

36. Positron Emission
Positron particle – is essentially an electron that has a positive charge.
A proton can be converted into a neutron by emitting a positron 11p n b
A useful way to decrease the number of protons.
Represented by the symbol: +10e or +10b

37. Positron Emission Example: 1938K 1838Ar + +10b
Notice that the atomic number decreases by one but the mass number stays the same.
Can be viewed as the opposite of beta emission.

38. Half-Life
Half-life - the amount of time it takes for one-half of a radioactive sample to decay is called the half-life of the isotope
It is given the symbol: t1/2
No two radioactive isotopes decay at the same rate

39. Half-Life
Useful application of half-life is radioactive dating. Used to determine the age of things.
Carbon-14 (t1/2 – 5715 years) dating can be used to determine the age of something that was once alive.
Examples include animal and plant species.
Cannot be used to determine the age of rocks.

40. Half-Life Radium-226 has a half-life of 1599 years.
Half of a given amount of radium-226 decays in 1599 years.
In another 1599 years, half of the remaining radium-226 will decay.
This will continue until there is a negligible amount of radium-226 remaining.

41. Half-Life
Decay of radium-226

42. Half-Life Each radioactive element has its own half-life.
More stable elements decay slowly and have longer half-lives.
Less stable elements decay quicker and have shorter half-lives

43. Radioactive Decay Rates

44. The time required for half of a sample to decay
Half-Life
The time required for half of a sample to decay

45. Half-Life

46. Half-Life
Problem:
Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope.
First determine the number of half-lives that have elapsed!

47. Half-Life
Answer:
First determine the number of half-lives that have elapsed!
57.2 days / 14.3 days = half lives
4.0 mg mg

48. Classwork Review HW Page 709 Problems 1, 3, 4 and 6 Page 724

49. Decay Series
Some nuclides (particularly those Z>83) cannot attain a stable, nonradioactive nucleus by a single emission.
The product of such an emission is itself radioactive and will undergo a further decay process.
Heavy nuclei may undergo a whole decay series of nuclear disintegrations before reaching a nonradioactive product.

50. Trying To Reach Nuclear Stability
Decay Series – a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached.
The heaviest nuclide of each series is called the parent nuclide.
The nuclides produced are called daughter nuclides.

52. The Four Known Decay Series

53. Artificial Transmutations
Transmutation – a change in the identity of a nucleus (element) as a result of a change in the number of its protons.
Artificial radioactive nuclides are radioactive nuclides not found naturally on Earth.
Artificial Transmutation – bombardment of stable nuclei with charged and uncharged particles (protons, alpha particles and neutrons)

54. Artificial Transmutations
Great quantities of energy are required to bombard nuclei with these particles.
These reactions are usually done by accelerating the particles in a high magnetic or electrical field.
The instrument used is a particle accelerator.

55. Artificial Transmutations
Used to produce all the elements in the periodic table with more than 92 protons in their nucleus.
Also used to produce technetium (43 protons), astatine (85), francium (87) and promethium (61).

56. Examples of elements created by artificial transmutation

57. Homework
Page 724
Problems

58. Section 22-4
Nuclear Fission
and
Nuclear Fusion

59. Objectives
Define the terms nuclear fission, chain reaction and nuclear fusion.
Explain how a fission reaction is used to generate power.
Discuss fusion reactions.

60. Nuclear Fission
Nuclear Fission – a very heavy nucleus splits into more stable nuclei of intermediate mass.
This process releases an enormous amount of energy.
Can occur spontaneously or when nuclei are bombarded with particles.

61. Nuclear Fission Example: Uranium-235
When uranium-235 is bombarded with neutrons, the uranium nuclei becomes unstable.
The nucleus splits into medium-mass parts with the emission of more neutrons.
The mass of the products is less than the mass of the reactants. The missing mass is converted to energy.

63. Example: Uranium-235
92235U n Ba Kr n
The product are barium, krypton and
3 more neutrons.
The reaction is balanced.

64. Nuclear Fission
When fission of an atom bombarded by neutrons produces more neutrons, a chain reaction can occur.
Chain Reaction – a reaction in which material that starts the reaction is also one of the products and can start another reaction.
In the last slide neutrons are generated from the initial reaction and can start a second reaction.

65. The chain reaction continues until all of the uranium-235 atoms have split or until the neutrons fail to strike uranium-235 nuclei.
Critical Mass - the minimum amount of nuclide that provides the number of neutrons needed to sustain a chain reaction.
Atomic Bombs – uncontrolled chain reactions.
Nuclear Reactors – controlled chain reactions.

66. Nuclear Fission
Only two fissionable isotopes are used during nuclear reactions.
Uranium-235 and plutonium-239
Uranium-238 (the more abundant isotope) only produces 1 neutron. This will not sustain a chain reaction.
92238U n Ba Kr n

67. Atomic Bombs
In an atomic bomb, two pieces of a fissionable isotope are kept apart. Each piece by itself is not going to explode.
When time comes for the bomb to explode, conventional explosives force the two pieces together to cause a critical mass.
The resulting chain reaction is uncontrolled, releasing a tremendous amount of energy.

68. Nuclear Power Plants
The secret to controlling a chain reaction is to control the neutrons.
If the neutrons can be controlled, then the energy produced can be controlled.
That is what takes place with nuclear power plants.

69. Nuclear Power Plants
In many respects, a nuclear power plant is similar to a conventional power plant (coal, oil, natural gas).
In these power plants the fuel is burned and the heat produced is used to boil water, which in turn is used to make steam.
The steam is then used to turn a turbine that’s attached to a generator that produces electricity.

70. Nuclear Power Plants
In a nuclear power plant the heat is produced through a fission reaction.
The fissionable isotope (uranium-235) is contained in fuel rods in the reactor core.
Control rods made of boron or cadmium (not fissionable) are also in the reactor core.

71. Nuclear Power Plants
The control rods act like neutron sponges to control the rate of radioactive decay of the uranium-235.
Operators can stop a chain reaction by pushing all the control rods into the reactor core, where they absorb all the neutrons.
Operators can pull out the control rods slowly to produce the desired amount of heat.

72. Nuclear Power Plants
Water is circulated through the reactor core, and the heat generated by the fission reaction is absorbed
The water flows to a steam generator where steam is produced.
The steam then is used to turn the turbine that generates electricity.
The water is recycled in a closed system.

73. Nuclear Reactor

74. Nuclear Fusion
Nuclear Fusion – light-mass nuclei combine to form a heavier, more stable nucleus.
Nuclear fusion is essentially the opposite of nuclear fission.
Nuclear fusion releases more energy than nuclear fission.

75. Nuclear Fusion
Nuclear fusion reactions are what power the sun for light and heat.
4 11H He b

76. Nuclear Fusion
Uncontrolled fusion reactions of hydrogen are the source of energy for the hydrogen bomb.
A fission reaction is used to provide the heat and pressure necessary for the fusion reaction.
A hydrogen bomb is about 1,000 times more powerful than an atomic bomb.

77. Nuclear Fusion
Nuclear fusion: the goal of scientists has been to control a fusion reaction.
Fusion reactions can provide an unlimited supply of energy that has no nuclear waste.
The product from a fusion reaction is helium, already found in our atmosphere.

78. Nuclear Fusion Three major obstacles need to be solved:
Temperature - The estimated temperature required to start the reaction is 40,000,000K.
Time – nuclei must be held together long enough.
Containment – At the required temperature all known material would vaporize.

79. Classwork Distinguish between nuclear fission and fusion
Define chain reaction
Explain how fusion can be a good source of energy

80. Review Classwork
Page 724
Problems:

81. End of Chapter

82. Radiation Energetics Alpha Particles Beta Particles
relatively heavy and doubly charged
lose energy quickly in matter
Beta Particles
much smaller and singly charged
interact more slowly with matter
Gamma Rays & X-rays
high energy
more lengthy interaction with matter

84. Hazards of Radiation Types
Alpha Emissions
easily shielded
considered hazardous if alpha emitting material is ingested or inhaled
Beta Emissions
shielded by thin layers of material
considered hazardous if a beta emitter is ingested or inhaled
Gamma Emissions
need dense material for shielding
considered hazardous when external to the body

85. The Radon Story

86. Radon-222
Originates from U-238 which occurs naturally in most types of granite
Radon-222 has a half-life of days
It decays via alpha emissions
This isotope is a particular problem because it is a gas which can leave the surrounding rock and enter buildings with the atmospheric air